All High School Chemistry Resources
Example Question #1 : Help With Enthalpy
Consider the following balanced reaction:
What is the change in enthalpy if of solid carbon is used in the above reaction?
The enthalpy of describes the amount of heat when the amount of carbon in the balanced reaction (two moles) is used. Since only of carbon are used, we can find how much heat is released.
When two moles of carbon are used, are released. Two moles of carbon is equal to of carbon, based on carbon's atomic mass.
Knowing this, we can set up proportions in order to determine how much heat is released by of carbon.
So of carbon results in of heat being released to the surroundings.
Example Question #2 : Help With Enthalpy
Consider these two half reactions:
Based on these half reactions, find the enthalpy for the following reaction:
Hess's law states that the enthalpy of the total reaction is equal to the enthalpy of the steps required to get to the total reaction, regardless of the path that is chosen. This means that we can combine the two half steps with known enthalpies in order to solve for the enthalpy of the main reaction.
Since step 1 results in two moles of liquid water, we need to use the second step twice in order to replace them with two moles of water vapor.
Since the total reaction is created by step 1 occurring once and step 2 occurring twice, we can write the enthalpy as:
Use the given enthalpies of the steps to calculate the total change in enthalpy.
Example Question #3 : Help With Enthalpy
How much energy is required to heat of from to ?
Use the following formula:
Plug in values: